Microprocessors and other integrated circuits typically require stable clock signals for their internal operations. In very early integrated circuit microprocessors, these signals were provided by circuits external to the microprocessor, typically in the form of a second integrated circuit dedicated to that purpose. In more recent designs, the clock signal generating circuits are integrated onto the same chip as the microprocessor or other device. In either case, the clock signal generating circuits typically rely upon an externally-provided crystal oscillator to provide the reference frequency upon which the clock signal is based.
The generation of a stable, known duty-cycle clock signal from the signal provided by the external crystal oscillator is accomplished by any of several known means. Most commonly, an inverter circuit of the general type disclosed in U.S. Pat. No. 3,676,801 is used in conjunction with the crystal to produce an acceptable clock signal. This clock signal may be divided down to the desired frequency of the clock signal for the microprocessor or other circuit. In some cases, a phase locked loop (PLL) frequency synthesizer circuit is used in addition to the basic input circuit. The PLL circuit typically comprises a phase comparator, a filter, a voltage controlled oscillator (VCO) and a divider chain. An example of a microprocessor with such a clock signal generating circuit is the MC146805H2, which is available from Motorola, Inc. of Austin, Tex.
A problem which occurs in integrated circuits which rely upon an external crystal oscillator as a clock signal reference, and particularly in microprocessors, is that of loss of the crystal signal. Through mechanical failure or other causes, the frequency reference signal produced by the crystal oscillator may be lost. This causes the microprocessor to stop operating suddenly. For instance, if the microprocessor is performing engine control functions in a vehicle, the engine may stop suddenly and, perhaps, place the occupants of the vehicle in some danger. Problems such as loss of the crystal reference signal become more important as microprocessors are increasingly used in harsh environments, such as that of an under-hood engine management system.
The present invention operates using a wider range of reference frequencies than those normally used by prior analog techniques. Prior analog techniques for detecting a loss of clock signal have the disadvantage that they do not generally function adequately using low frequency oscillators because the analog circuit components required (i.e. a very large capacitor and a very small current source) are difficult to incorporate on a microcomputer integrated circuit. The present invention, however, provides a digital circuit which is useful for detecting the loss of a clock signal across the entire range of possible reference frequencies without a significant degradation of performance at lower frequencies.